1. Field of the Invention
The present invention relates to a method for controlling a stage apparatus, and more specifically, though not exclusively, controlling a stage apparatus by using a planar motor.
2. Description of the Related Art
An exposure apparatus includes a wafer stage for facilitating the placement of a wafer thereon and to move the wafer. To drive such a stage, the use of a planar motor has received attention in recent years. FIGS. 1, 2A, and 2B illustrate a wafer stage using a planar motor as discussed in U.S. Pat. No. 6,987,335 (corresponding to Japanese Patent Laid-Open No. 2004-254489).
In FIGS. 2A and 2B, a plurality of magnets are disposed on a lower surface of a stage (movable element), and a plurality of coils are disposed on a base (stator). The stage is moved with respect to the base by the Lorentz force generated by energizing the coils through which magnetic flux generated by the magnets passes.
A mirror is disposed on the stage, and the position of the stage is measured by causing measurement light emitted from an interferometer disposed outside the stage to be reflected by the mirror.
The stage apparatus using the planar motor described above has no guide. Therefore, in the case where a positional servo for the stage becomes inoperative, for example, when power is turned on in the apparatus, power fails, or power is shut down due to an emergency, the stage can be stopped at any angle of rotation about an axis substantially orthogonal to the base. However, if the stage is in a largely rotated state, it is difficult to detect the position of the stage using the interferometer. This is because the optical path of light from the interferometer is changed by being reflected by the mirror largely tilted together with the stage and thus the light does not return to the interferometer.
If the position of the stage is not detected, the stage cannot be subjected to the positional servo. In this case, the rotation of the stage is required to be manually returned to a state at which the position of the stage is detectable. Automatic positional determination would be useful in terms of both maintenance and throughput.